A fuel cell stack is a stack of fuel cells (unit cells), each consisting of a membrane electrode assembly (hereinafter also referred to as “MEA”) and a pair of separators sandwiching the MEA. The MEA includes a polymer electrolyte membrane and a pair of catalyst electrodes which sandwiches the polymer electrolyte membrane.
The polymer electrolyte membrane is composed of an electrolyte which contains a polymer ion-exchange membrane or the like, such as a sulfonic acid group-containing fluorine resin ion-exchange membrane or hydrocarbon resin ion-exchange membrane.
The catalyst electrode is composed of a catalyst layer that promotes a redox reaction therein and of a gas diffusion layer having both air permeability and electric conductivity. The catalyst layer is in contact with the polymer electrolyte membrane. The gas diffusion layer is composed of a carbon coat layer for improving adhesion to the catalyst layer and of a gas diffusion base layer through which a gas supplied from an external source is allowed to diffuse to the catalyst layer. The catalyst layer for the fuel electrode contains, for example, platinum or platinum-ruthenium alloy, and the catalyst layer for the air electrode contains, for example, platinum or platinum-cobalt alloy.
The separator is a conductive member for avoiding mixing of a fuel gas to be supplied to the fuel and an oxidizing gas to be supplied to the air electrode.
In a fuel cell stack, unit cells can be electrically connected in series by stacking them on top of each other. Such a fuel cell stack further includes end plates for sandwiching the cell assembly (see, e.g., Patent Documents 1 and 2). In some cases, in order to apply a uniform load to the cell assembly, a spring module (see, e.g., Patent Documents 3 and 4) or an elastic member (see, e.g., Patent Documents 5 and 6) is disposed between the cell assembly and the end plate.
FIG. 1 is a cross-sectional view of fuel cell stack 1 described in Patent Document 6. As illustrated in FIG. 1, fuel cell stack 1 disclosed by Patent Document 6 includes cell assembly 12, pressure plates 2 and 8 which sandwich cell assembly 12, and end plate 20. Elastic members 19 are disposed between end plate 20 and pressure plate 2.
The pressure plates are made electrically conductive on their surface which faces cell assembly 12, with the other surfaces being made insulating. Elastic members 19 are each held by concaved portion 9 formed in end plate 20 and by concaved portion 7 formed in collector plate 2.
Electric energy can be extracted by supplying a fuel gas (hydrogen is contained) and an oxidizing gas (oxygen is contained) to the respective unit cells of a fuel cell stack configured as described above. The following describes chemical reactions that occur by supplying a fuel gas and an oxidizing gas to the unit cells.
Hydrogen molecules supplied to the fuel electrode are split into hydrogen ions and electrons in the catalyst layer. The hydrogen ions migrate through the humidified polymer electrolyte membrane to the air electrode side. On the other hand, the electrons migrate through an external circuit to the air electrode to which oxidizing gas is supplied. The electrons migrating through the external circuit can be utilized as electric energy. In the catalyst layer of the air electrode, hydrogen ions from the polymer electrolyte membrane, electrons from the external circuit, and oxygen supplied to the air electrode are reacted together to form water. In addition, heat is generated during the reaction.
By supplying a fuel gas and an oxidizing gas to fuel cells in this way, it is possible to obtain electric energy and thermal energy at the same time. This allows fuel cell stacks to be used as a household co-generation system that requires both power generation and hot-water supply (see, e.g., Patent Document 7). In household co-generation systems, heat generated during power generation is recovered using a coolant that is caused to flow through channels formed in the separators. The recovered heat is stored in a hot-water storage tank for subsequent utilization as thermal energy.
In addition, technologies are known in which the cell assembly-side surface of end plates are concaved or convexed for the purpose of facilitating temperature adjustment of the fuel cell stack during operation (see, e.g., Patent Document 8).                [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-123114        [Patent Document 2] Japanese Patent Application Laid-Open No. 08-203553        [Patent Document 3] Japanese Patent Application Laid-Open No. 2004-288618        [Patent Document 4] U.S. Patent Application Publication No. 2005/0277012        [Patent Document 5] Japanese Patent Application Laid-Open No. 2007-257865        [Patent Document 6] Japanese Patent Application Laid-Open No. 2006-179220        [Patent Document 7] Japanese Patent Application Laid-Open No. 2008-293996        [Patent Document 8] U.S. Patent Application Publication No. 2008/0090122        